Still no go although the datasource exceptions are fixed now (changed SELECT 1 to SELECT NULL FROM DUAL). I attached the server.log file which shows some exceptions. The first one that shows up is
Caused by: java.io.InvalidClassException: org.drools.rule.MVELDialectRuntimeData; local class incompatible: stream classdesc serialVersionUID = 5166066541265170937, local class serialVersionUID = -64624826351992706
I also found this one:
14:03:02,281 ERROR [stderr] (Thread-17) java.lang.IllegalArgumentException: Named query not found: TasksAssignedAsPotentialOwnerByStatusByGroup
The TaskORM.xml file in the jbpm-human-task-core-5.3.0.Final.jar in the war file has been replaced with the one out of the latest 5.4.0 snapshot.